The present invention relates to an image forming apparatus, an image forming method and an image processing method for effecting recording on a recording material wherein a density of non-uniformity is attributable to a variation of a recording property among a plurality of recording elements of a recording head. More particularly, it relates to an image forming apparatus, an image forming method and an image processing method wherein moire or the like is attributable to error in a mounting position of the recording heads.
An ink jet recording apparatus is known in which a recording head provided with a plurality of ink ejection outlets, is an example of an apparatus using a recording head that is provided with a plurality of recording elements.
In such an apparatus, sizes and/or positions of the dots provided by the ink are not uniform due to variations in ejection outlet diameters of the ejection outlets and/or ejecting directions, and if this occurs, the printed image density is not uniform as well. Particularly, in a recording device of a serial type in which the recording head is scanningly moved in a direction that is different from the direction of the arrangement of the recording elements, for example, perpendicular thereto, the density of non-uniformity that is attributable to the above-described variation in the ejection outlet diameters results in stripes in the printed image, with the result that the quality of the image deteriorates.
In order to correct such a density non-uniformity, it has been proposed that in the image formation using a recording head of an ink jet recording type, one pixel or pixels of a line corresponding to one scanning of a recording head, is printed by ink ejected from different ejection outlets on the basis of image data which have been processed for low gradation. This can be done by feeding the sheet through a distance smaller than the width of the recording head and completing one pixel by a plurality of scans for paths.
FIG. 3 shows an arrangement of a conventional ink jet recording head. The printer usable with this recording head forms an image by CMYK inks (four colors). The recording head 601 is provided with two ink jet heads of each ink color, and a head 603 (rear head) disposed at an upper position in the Figure and a lower head 602 (front head) are disposed in a sub-scan direction with a distance of 2.5 bands (one band is a unit of a width measured in a direction of the nozzle array operated in one scan of the ink jet head).
FIG. 4 shows an overlaying state of printing using the head 602 and the rear head 603 in the printer using the recording head show in FIG. 3. One sub-scan is carried out for one main-scanning. A distance of feeding in the sub-scan direction is one band, so that image is formed with deviation of half-band between the front head 602 and the rear head 603. Here, the half-band 703 is constituted by the uproar one half of the front head 602 and the lower one half of the rear head 603, and the half-band 704 is constituted by the lower one half of the front head 602 and the upper one half of the rear head 603.
Referring to FIG. 5, the description will be made as to the process in which the image data of a multi-level type fed to the printer are binarized, and are converted to head driving data to eject the ink from the nozzle.
(1) The image data of the multi-level type transferred from a host computer is stored in an image data storing apparatus 801. The data are fed out from here one by one band.
(2) A pallet conversion circuit 802 separates the image data to multi-level data of respective ink colors. The description will be made as to black ink Bk as a representative.
(3) A xe2x80x9cgammaxe2x80x9d conversion circuit 803-K effects a xe2x80x9cgammaxe2x80x9d conversion to the multi-level data separated for each ink color.
(4) A non-uniformity correcting circuit 804-K corrects the non-uniformity due to the variation in the properties of the nozzles, using a non-uniformity correction table (look-up table for conversion from multi-level data to multi-level data).
(5) A binarizing circuit 805-K coverts the multi-level to binary data using an error diffusion method (ED).
(6) A SMS (sequential multi-scanning) circuit 806-K determines which one of the front head 602-K and the rear head 603-K is to be used. The SMS circuit, when a certain raster scan is considered, allots the data to the front, rear, front, rear, namely, alternatingly from the left end dot, and they are outputted to the TMC (Timing Memory Controller) circuits 807-K1, 807-K2. By doing so, it does not occur that adjacent dots are printed by the same head, and the printing operation can be carried out at twice the speed of the driving frequency of the head. The dot appearing first in each raster scan is printed by the rear head 603-K in the case of an odd number raster scan and by the front head 602-K in the case of an even number raster scan.
(7) In the TMC circuits 807-K1, 807-K2, the data for one band are outputted to each head 602-K, 603-K. A positional deviation in the main-scanning direction between the heads 807-K1, 807-K2 is adjusted using a lateral registration adjusting value, where the output timing for one array is different depending on the lateral registration adjusting value.
(8) PHC (Printer/head connector) substrates 808-K1, 808-K2 output the binary data in the nozzle array direction, corresponding to the nozzles which actually effect printing. A positional deviation in the nozzle array direction between heads 807-K1, 807-K2 is adjusted by the longitudinal registration adjusting value. The recording head in this example has 1344 nozzles and additional upper and lower 8 nozzles which are effective for printing, and therefore, the longitudinal registration adjusting value is in the range of xe2x88x928-+8. When the longitudinal registration adjusting value is xc2x10, central 1344 nozzles are used, but when the longitudinal registration adjusting value is xc2x11-8, the actually used nozzles are deviated by 1-8 nozzles from the center. The data for 1344 nozzles are outputted corresponding to the nozzles to be actuated, using the longitudinal registration adjusting value.
(9) Finally, the binary data for each nozzle are converted to head driving data by a print control device (Head CPU) 809 to eject the ink for printing.
Referring to FIG. 7, there is shown the processes (5) and, (6) in more detail. The multi-level data after the color separation, the xe2x80x9cgammaxe2x80x9d conversion and the non-uniformity correcting process (802, 803, 804 in FIG. 5) are subjected to an error diffusion process (FIG. 7B) using an error diffusion matrix A (FIG. 6A) to effect binarization (FIG. 7C). In FIG. 6, the asterisk indicates the noting pixel. By SMS (806 in FIG. 5), the determination will be made as to whether the front head or the rear head is to be used. The data shown in FIG. 7E is fed to the front head, and the data shown in FIG. 7F are fed to the rear head.
According to the above-described method, an image in the predetermined region is formed by different nozzles of two heads, and therefore, the density non-uniformity or the like due to variation in the properties of nozzles can be reduced. In addition, the pitch of the interfaces is half-band so that bandings is less conspicuous.
In this manner, the density non-uniformity attributable to the properties peculiar to the ejection outlets of the ink jet head can be diffused on the recording material, by which the density non-uniformity is reduced. This is called xe2x80x9cmulti-scanxe2x80x9d or xe2x80x9cmulti-pathxe2x80x9d. Furthermore, a so-called sequential multi-scan (SMS) system has been proposed in which the ink ejection outlets are actuated in a predetermined order mainly to make the ink ejection outlets uniform.
However, it has been found that there is a point to improve in use with a large scale multi-color ink jet printing apparatus. For example, a textile printing apparatus in which the scanning range is as large as several meters, and in addition to the yellow color, magenta color, cyan color and black color inks, light color and special color inks are used. More particularly, if an ideal apparatus is used with the method, the recorded pixels are uniformly arranged. However, practically, the size and/or deposit positions of the dots provided by the ink varies due to the variation of the ejection outlet diameters of the ink and the variation of the direction of ejection, and in addition, due to unavoidable error in the mechanical mounting accuracy among recording heads, the variation in registration occurs between main-scanning. Therefore, the intervals between recorded unit pixels to be overlaid by the multi-scan are different with the result of moire and/or non-uniformity with half-scan interval. Particularly, in the case of reciprocal main-scanning recording, the mounting angle of the recording head or the shape of the ejection outlet is different between the forward path and the backward passage with the result being reciprocation non-uniformity.
The inventor""s investigations have revealed that plurality of scans are in complete complementary relation, and this is a cause of the problem. In the above-described method, the half-band images are complementary with each other, that is, the dots provided by the scans are spatially complementary with each other. Therefore, if an error in registration occurs between the bands, for example, when the lateral registration is deviated by a half dot, the complementation is effected with the lateral registration remaining.
If the recording is effected using an ideal apparatus and the method, the solid image has uniformly distributed dots without overlapping. In FIG. 8, designated by 21 (black dot) are the dots provided by a first scanning, and designated by 22 (white dot) are the dots provided by the second scanning.
However, in the actual machines, the unavoidable error in the physical accuracy results in small change or non-uniformity in the respective scans, and therefore, when the dots provided by prospective scans are overlaid, the dots are too close or too remote relative to each other. For example, when a lateral registration of a half dot occurs in the first scanning, sparse/dense states or overlapping of dots appear, as shown in FIG. 9.
Designated by 21 (black dots) are the dots printed by the first scanning, and designated by 22 (white dots) are the dots provided by the second scanning. As a result, the printed image is different from the intended image in the image density, that is, the image density is not uniform. More particularly, dark bands and light bands appear (xe2x80x9cbandxe2x80x9d is a unit width in the direction of a nozzle array covered by one scan of the ink jet head).
Accordingly, it is a principal object of the present invention to provide an image forming apparatus, an image forming method and an image processing method wherein the multi-scan type is used, and a high quality image can be provided at high speed with a reduced density difference due to a difference between the unit pixels recorded.
It is another object of the present invention to provide an image forming apparatus, an image forming method and an image processing method wherein the image density does not significantly change even when the registration slightly changes due to a physical error, so that uniform images can be provided with reduced density non-uniformity.
According to an aspect of the present invention, there is provided an image forming apparatus wherein an image for a predetermined region of a recording material is formed using images having a complementary relation by a plurality of scans of a recording head, said apparatus comprising allocating means for allocating a multi-level image data for the predetermined region for the scans; gradation reducing means for reducing gradation of the multi-level image data allocated by said allocating means, respectively; image forming means for forming an image having the complementary relation by driving said recording head in said scans on the basis of the image data having gradations reduced by said reducing means; wherein the complementary relation of the image by said forming means is reduced by at least one of said allocating means and said gradation reducing means.
With this structure, the images provided by the respective scannings have less complementary relation or reduced complementary relation so that even if the registration changes are due to a physical accuracy error, the image density does not change significantly since the dependency upon the registration is less, and therefore, the uniformity can be maintained.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.